In commonly owned U.S. Pat. Ser. No. 726,138, which is a continuation of U.S. Pat. Ser. No. 556,692, filed on Jul. 20, 1990 by Shaw et al, entitled "Dispenser Mechanism", there is disclosed a work station comprising a dispensing pump mechanism that automatically raises and rotates a dispenser from one position to another. This is particularly useful in a blood analyzer. The mechanism features a dispenser and a cam follower mounted for vertical movement on an outer carrier. The carrier frictionally engages a cam track that is formed on a cam that fits inside the carrier and is rotated by a pinion gear. As long as the carrier and the cam rotate together, the pump dispenser rotates without changing elevation. However, when the outside carrier hits one of two mechanical stops, it ceases to rotate while the cam continues to rotate, forcing the cam follower and dispenser to move along the cam track. The change in vertical height along the cam track produces the vertical motion of the dispenser.
This mechanism has been found to work quite well. However, to arbitrarily hold the carrier against rotation, such as when maintenance is needed, a ring of teeth is used on the carrier, to be engaged with a solenoid--activated lever. The solenoid is an additional mechanism that adds cost and maintenance to the mechanism. Therefore, prior to this invention, there has been a need to improve the mechanism by eliminating the need for the solenoid and its lever, while retaining the function of being able to lock the carrier against inadvertent rotation.